Randomized clinical trial to assess the protective efficacy of a Plasmodium vivax CS synthetic vaccine

Abstract

A randomized, double-blind, controlled vaccine clinical trial was conducted to assess, as the primary outcome, the safety and protective efficacy of the Plasmodium vivax circumsporozoite (CS) protein in healthy malaria-naïve (phase IIa) and semi-immune (phase IIb) volunteers. Participants (n = 35) were randomly selected from a larger group (n = 121) and further divided into naïve (n = 17) and semi-immune (n = 18) groups and were immunized at months 0, 2, and 6 with PvCS formulated in Montanide ISA-51 adjuvant or placebo (adjuvant alone). Specific antibodies and IFN-γ responses to PvCS were determined as secondary outcome; all experimental volunteers developed specific IgG and IFN-γ. Three months after the last immunization, all participants were subjected to controlled human malaria infection. All naive controls became infected and drastic parasitemia reduction, including sterile protection, developed in several experimental volunteers in phase IIa (6/11) (54%, 95% CI 0.25-0.84) and phase IIb (7/11) (64%, 95% CI 0.35-0.92). However, no difference in parasitemia was observed between the phase IIb experimental and control subgroups. In conclusion, this study demonstrates significant protection in both naïve and semi-immune volunteers, encouraging further PvCS vaccine clinical development. Trial registration number NCT02083068. This trial was funded by Colciencias (grant 529-2009), NHLBI (grant RHL086488 A), and MVDC/CIV Foundation (grant 2014-1206).

Conflict of interest statement

The authors declare no competing interests.

© 2022. The Author(s).

Figures

Fig. 1. Study flow diagram.

The study flow diagram describes the number of individuals in the screening, immunization, and CHMI steps. From 121 volunteers who initially accepted screening (38 naive, 83 semi-immune), 86 were excluded or declined participation and 35 were enrolled. Seventeen were naive and were allocated to Phase IIa study [12 Experimental (Exp) + 5 Control (Ctrl)], and 18 semi-immune to the Phase IIb study (13 Exp + 5 Ctrl). All 35 volunteers (age range 19–44 years) were immunized with PvCS LSP or placebo formulated in Montanide ISA-51. Two volunteers withdrew after the first immunization, and one more (semi-immune) was dropped out because of diabetes mellitus diagnosis.

Fig. 2. Amino-acid sequences of the N, R, and C fragments.

Sequence and localization of the three P. vivax CS LSP (N, R, and C) fragments used as immunogens. The N polypeptide corresponded to N-terminal amino acids (aa) 20–96 (N-term), and the C peptide to C-terminal aa 301–372 (C-term). The R peptide VK210 (type I) corresponded to a construct based on the first central repeat (aa 96–104) in tandem three times and collinearly linked to a universal T-cell epitope (ptt-30) derived from tetanus toxin. For the first dose a peptide mixture of N-terminal (term) and fragments (1N:1C) (50 μg/each peptide) was used, whereas for the second and third doses the peptide mixture included N-term, C-term, and R fragments (1N:1C:1R) (50 μg/each peptide).

Fig. 3. Anti-PvCS LSP antibody response in naive and semi-immune volunteers.

Kinetics of specific IgG response to N, R, and C fragments in both Exp (n = 11) and Ctrl (n = 5) volunteers in Naive (a) and Semi-immune groups (b). Sterilely protected volunteers are shown in red lines and non-protected in blue lines. Symbols indicate IgG titer as log10 of ELISA values throughout the 10 months of the study. Black triangles at the bottom of the figure indicate immunizations (at 0, 2, and 6 months) and black arrows the controlled human malaria infection (CHMI, at month 9). Significant boosting of antibodies to the three fragments was higher in the naive than in the semi-immune groups (N-, p = 0.046; R-, p = 0.0013; C-, p = 0.00505). Antibody titers did not associate with infection intensity in the naive group (N, p = 0.72; C, p = 0.55; R, p = 0.65) or semi-immune group (N, p = 0.98; C, p = 0.73; R, p = 0.52). Source data are provided as a Source data file.

Fig. 4. Specific induction of IFN-γ by PvCS-LSP in naive and semi-immune groups.

Single-cell interferon gamma (IFN-γ) ex vivo production by fresh peripheral blood mononuclear cells (PBMC) from naive (n = 17) and semi-immune (n = 18) volunteers collected before immunization (0, 2, 6 months) and CHMI (month 9). Values are expressed as mean of IFN-γ (spot-forming cells) sfc/log106 in response to 40 h of in vitro stimulation with each PvCS protein fragment (N, R, C). Cells produced IFN-γ upon stimulation with the different fragments in an unstable manner throughout the study phases but with significant differences between naive and semi-immune volunteers (N, p = 0.046; R, p = 0.0013; C, p = 0.005. Only peptides N (p = 0.0188) and R (p = 0.0060) displayed significant differentes between first and third immunization. Neither naive (N, p = 0.67; C, p = 0.94; R, p = 0.15) nor the semi-immune (N, p = 0.76; C, p = 0.71; R, p = 0.48) IFN-γ levels were associated with parasitemia. Red symbols denote sterilely protected volunteers. Source data are provided as a Source data file.

Fig. 5. Survival curve for naive and semi-immune groups.

Protective efficacy Kaplan–Meier curves are shown for naive (a) and semi-immune (b) groups. One volunteer of the semi-immune Ctrl group (red line) did not develop parasitemia. Exp: Experimental (blue lines), Ctrl: Control. Prepatent periods of the Exp naive (15.8) and semi-immune (17.6) groups presented significant differences (p = 0.0034). Source data are provided as a Source data file.